The present invention is directed to a communications network, and more particularly to a fully meshed network of personal communications terminals.
As a communications medium, satellite has the unique capability of providing multipoint-to-point (multiple access) and point-to-multipoint (broadcast) mode communications simultaneously, thereby permitting multipoint-to-multipoint communications by small users. However, current satellite systems designed for small users are limited in network topology as well as applications. For example, VSAT (very small aperture terminal) data networks and satellite mobile networks (e.g., INMARSAT) are all limited to star network topology where small users are communicating into a large hub or gateway station. Any communications between remote small users must be relayed by the central hub or gateway station, and hence results in a double-hop mode of transmission over the satellite transponders. While double-hop long delay might be marginally acceptable to some low speed data transmission, it is totally unacceptable for two-way voice communications. Therefore, the present VSAT as well as INMARSAT mobile networks are not suitable for communications between small users.
Code Division Multiple Access (CDMA) is a spread spectrum communications technique whereby multiple stations communicate with one another over a common link, with each station being assigned a unique PN code to be used in "spreading" the bandwidth of signals destined for that station from other stations. Each station receives all signals over the link, and looks for correlation with its respective unique PN code. Signals "spread" with a station's unique PN code will exhibit a high correlation and can therefore be "de-spread" using the same code.
CDMA is advantageous in that it uses available bandwidth efficiently, but the very concept of spread spectrum communications requires avoidance of signals with high periodicity, e.g., synchronizing signals. Thus, elaborate measures must be taken to ensure synchronization between the various stations on the network, and the cost of the system becomes very high. For this and other reasons, CDMA has been limited to use in star network topologies where synchronization is easiest, such as Omnitrack, Qualcomm digital cellular mobile, and Equatorial systems. There are no applications of this technology to a "fully-meshed" network topology, i.e., where there is a direct connection path from any node on the network to any other node on the network.
In addition, small user communications networks typically involve significant intermittent communications, and efficient utilization of bandwidth would dictate that the bandwidth be allocated on a demand assigned basis and used in a "burst" mode. However, using burst mode communications has conventionally required that synchronization be acquired very rapidly at the beginning of each burst, but this is impractical in a spread spectrum system where the transmission of highly periodic signals for synchronization purposes is to be avoided.
Accordingly, it is an object of the present invention to provide a fully meshed network of small personal communications terminals.